By: R. Koelewijn, May 2000

Long believed to have been extinct almost 60 Million years ago the coelacanth (pronounced see-la-kanth) caught the imagination of the public when a live specimen was caught off the coast of South Africa in 1938. The living species was called Latimeria chalumnae after its discoverer M. Courtenay Latimer and its finding place, nearby the river Chalumna.

The Coelacanth originated 400 million years ago from the same stock of fish that gave rise to the ancestors of land-dwelling vertebrates called tetrapods (including us). The most ancient Coelacanth that is known is Diplocercides, which is Late Devonian in age. The devonian coelacanth shows an extraordinary number of features of Latimeria chalumnae which tells us that the group as a whole has not evolved much since the Devonian. This lack of change enabled Professor J.L.B. Smith to identify the coelacanth caught in 1938 from a rough sketch, which Latimer made.

The first evidence of fishlike vertebrates is small fragments of bone from the Cambrian of North America. The fossil record then has a gap until the Mid-Ordovician, when remains of archaic fishes are found. These fishes belonged to an early radiation of vertebrates that had a simple mouth structure and feeding mechanism, they lacked teeth and jaws.

Not until the late Silurian there is evidence of the origin and radiation of a more modern sort of vertebrate. This one has jaws and teeth and a full complement of fins, the ancestors of all modern vertebrates. By the end of the Silurian we can already find the first examples of all four of the major groups of jawed vertebrates.

These were:

• the Acanthodii (a group of small fishes that was extinct by the mid-Permian).
• the Placodermi (a group of heavily armored fishes that was extinct at the beginning of the Carboniferous).
• the Chondrichthyes (sharks and rays and their relatives).
• the Osteichthyes (the bony fishes).

The Osteichthyes includes two groups distinguished on the basis of their paired fins. these are the ray-fins (Actinopterygii) and the lobe-fins (Sarcopterygii). The bony fish comprises the great majority of living fishes. Only four genera of these are the lobe-finned sort namely the three lungfishes and Latimeria.

As the name "lobe-finned" suggests the group is characterized by a particular sort of fin structure that is not found in other groups of vertebrates. In the ray-finned fishes both the median and paired fins always have the same basic pattern of structure, each consisting of a number of bony, flexible fin rays, inserted directly into the body wall with skin stretching across them. the rays can be moved individually by muscles within the body wall. In lobe-fins by contrast the second dorsal, anal and paired fins are of a different type in which there is a muscular stomp ore "lobe" projecting from the body wall. This lobe has it’s own internal skeleton and muscles and the fin rays are restricted to a fan that attaches to the outer end of this lobe (see picture). The lobes give the fish a reptile like appearance and give the idea that the fish might actually be able to walk on its fins. These morphological features lead many scientists to believe the coelacanth lineage was the direct link to tetrapods, but recent molecular evidence suggests that lung fish might be more closely related to tetrapods.

Looking at the radiations of coelacanths in the fossil record something striking appears immediately. The group is an extremely ancient one in comparison with other groups of fishes yet at no time there were even more than a handful of species of genera around at one time. Their taxonomic diversity has always been very low. At the same time some genera and species have individually been extremely numerous and widespread.

In the Devonian, coelacanths were common but not really abundant, at least 20 different species excisted, nearly all of these were marine. The group was about equally abundant in the Carboniferous, but the Devonian species were then already extinct. During this period in the mississippian and pennsylvanian we can find coelacanths that must have been able to tolerate a full range of environmental conditions from salt to partially freshwater. An example is Rhabdoderma, a small coelacanth that is quite common in coal deposits in Europe and North America

By the Permian the whole group was almost extinct. But then we find an excellent record of Triassic coelacanths, around thirty species in all. This was the time that coelacanths reached a climax in types and numbers. One extremely abundant genus Diplorus was living in freshwater lakes and rivers of North America. During the Jurassic the group was down to fifteen or sixteen species, both freshwater and marine forms. In the Cretaceous there were fewer than ten. We find more marine forms, particularly in the great chalk deposits of Europe, these seem to be absent from the chalk deposits of North America.

The youngest fossil coelacanths are marine forms from the Cretaceous period: Macropoma a thirty centimeters large fish found from the Late Cretaceous of Europe and Asia. Here the story was thought to end.

The fact that there are no fossil coelacanths found from strata younger than the Late Cretaceous is probably explainable by both the actual distribution of coelacanths during that time and the conditions of preservation. It is very probable that the coelacanths that existed during the Cenozoic period were marine forms. The record of marine fishes from the Cenozoic period is modest and that of the marine fishes from deeper waters is very poor. Since the coelacanth could not compete for prey with modern fishes it probably retreated to depths where the others could not survive for lack of food. The only region in the world were the living coelacanth has been found is the Indian Ocean from which almost no suitable marine fossil record exists at all.

The first known habitat of the Coelacanth was found at the Comorans, a volcanic island group between Madagascar and the African continent. The Camoran Coelacanth is dark blue coloured with distinctive white flecks. They grow up to 1.5-2 meters in length and can weigh up to 70 kilograms. Females are observed to be larger than males.

The fins of the Coelacanth allow it to swim in every direction, including backwards and upside down. Coelacanth reproduce slowly and are proven to be Ovoviviparous (producing eggs that develop within the maternal body and hatch within the maternal body or shortly after extrusion from the parent). The dentition, jaw structure, and reduced gut length are adap-

tions for a predatory feeding behavior. The coelacanths were observed using a submersible and we know now that they live in lava caves, usually at depths between 100 and 400 meters.

By doing a rough count of the number of possible coelacanth caves along the west coast of Grand Comoro island and the number of fish per cave, plus the birth rate, Professor Hans Fricke and associates came up with an estimate for the total coelacanth population in the low hundreds. However, there is some question if this estimate takes into account the known but unexplored coelacanth population at the nearby island of Anjouan, the unseen juvenile population, and the new number of potential embryos.

A total population count is also complicated by another until recently unresolved issue: Is the Comorian population the only one? The Comoro islands are the only place where regular annual catches of 6-8 coelacanths occur at Grand Comoro and 4-5 at Anjouan. But from time to time coelacanths turn up elsewhere. It is not known whether these fish are strays from the Comoros or that they are representatives of satellite colonies.

The recent confirmed identification by Dr. Mark Erdmann of at least two specimens from North Sulawesi, Indonesia at 10,000 kilometers from the Comoros with no apparent water current interactions clearly proves that more colonies can be expected to be discovered. The Indonesian population appears to be distinct, although the specimens at least superficially resemble the Comoran coelacanths. The coelacanths found here are brown coloured and are smaller in size but they live at the same depths in caves along steep volcanic

While the Coelacanths have changed little through time, there have been modifications in structure in other forms of fishes so that the Coelacanth differs markedly in many ways from most modern bony fishes.

In the head of the Coelacanth quite massive bones lie right in the surface. The skull itself is not a single unit but is in two almost separate parts. an intercranial joint is thought to allow them to widen their gape when capturing prey.

Most bony fishes have a well developed soft air bladder, which lies above the intestines. The modern coelacanth has no true air bladder, it has a fat-filled buoyancy organ in the position in which the air bladder is generally found, this may have been a vestigial lung. In the coelacanth the gills are not soft cartilage like modern fishes but bony and hard. These bear teeth and not ordinary soft gill-rakers. The scales of the Coelacanth are characteristic. The main basal part is almost horny and with less bone than a comparable modern fish. The scales are thick, and lined with serrated rows of hardened denticles. The scales overlap so much that the whole body has a covering three scales thick, which creates a powerful protective armour.

The fins and tail are the most remarkable feature of the Coelacanth. The paired fins move in an alternating fashion which resembles a horse in a slow trot. The lobed fins have an internal bone structure (see above). The fin at the tip of the tail, called the epicaudal lobe, waves like a metronome. It is speculated that it may produce weak electric charges that help the fish maneuver though the darkness.

Three indentations on either side of the snout lead to a peculiar cavity, a jelly-filled rostral organ, which very likely functions as an electro-receptor to help in the location of prey (similar to that of sharks). Along the sides of the fish a pressure sensitive lateral line is well developed to sense the proximity of other fishes and surrounding structures.

The backbone or axial column is not of bone, but a hollow tube. An elastic unsegmented notochord substitutes for a backbone. This is how the coelacanth got its name. "Coelacanth" means "hollw spine". At the base of each fin there is a large and heavy bony plate, the "basal" plate, well known from fossils.

The Coelacanth Rescue Mission originated in the 1980's by members of several coelacanth expeditions to the Comoro Islands. Their mission is to help reduce coelacanth fatalities by distributing a Deep Release Kit to the local fishermen who accidentally catch about nine of the fish each year. Coelacanths caught by local fisherman live for only ten or so hours at the surface of the ocean. The cause of mortality is believed to be a combination of capture stress and over heating resulting in asphyxiation. (The surface waters are some 20 degrees warmer than where the coelacanth lives.)

The deep sea release kit is now the only direct means of conserving coelacanths. The coelacanth population in the Comoros is estimated to be in the low hundreds. Moreover, submersible research by the JAGO team from Germany (lead by Hans Fricke) suggests a 30-50% decline in the population size at Grand Comoro between 1991 and 1995. So the discovery of a second population is welcome news for the conservation of coelacanths. However, we know of only two confirmed coelacanths in Indonesia. Therefore, the population must be considered to be quite small.

Hans Fricke, "Coelacanths, the fish that time forgot", National geographic magazine, 173,

J.A. Musick, M.N. Bruton & E.K. Balon, "The biology of Latimeria chalumnae and evolution

Jerome F. Hamlin, www.Dinofish.com, "Coelacanth: The fish out of timeä ", ã 1999 Third

J.L.B. Smith, "Old fourlegs, The story of the Coelacanth", Longmans London, 1956, pages

John E. Mccoster, "The biology and physiology of the living coelacanth: Inferred natural

Keith S. Thomson, Living fossil; "The story of the Coelacanth", Hutchinson Radius, 1991,

Margaret M. Smith, "The biology and physiology of the living coelacanth: Influence of the

Roy Caldwell, Dr. Mark Erdmann, and Kirsten Lindstrom,

*Special thanks to Roy Caldwell of the Berkeley university for providing me with excellent photographs and allowing me to use them for my homepage: http://www.geocities.com/r_koelewijn

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